Several manufacturing and research activities require so-called "clean room" environments to avoid contamination of the articles being handled. It is standard practice to arrange a number of filtration modules in the ceiling area of a clean room, with some form of air outlet provided adjacent the floor level. Air is admitted to the filtration modules by standard ducting extending to a blower system, with the air normally being recirculated. The wide use of this type of equipment has developed a generally similar configuration for the modules, which includes some form of housing providing an equalization space between the housing intake and a filter element, and a grille defining the outlet of the module into the room. Some types of the module provide for the replacement of the filter elements from the interior of the room, and other forms of the modules are essentially throw-away assemblies that are discarded when the filter can no longer perform its function. In the latter case, it is common to utilize the wooden frame of the standard filter element as a part of the housing of the module. In both types of module, a grille of some form is mounted in a position spaced downstream from the outlet side of the filter. The standard filter element is the same in both cases, and consists of a rectangular wooden frame containing densely convoluted paper-like material separated by aluminum foil separation panels.
The filter element goes a long way toward the removal of particulate material suspended in the air flow, but can never remove all of the particles that might conceivably contaminate delicate instruments or electronic components. A valuable adjunct to clean room filtration equipment has been a type of air-ionization equipment that tends to neutralize any static charges that may develop on articles within the room so that they do not attract the suspended particles that still remain carried by the air stream, or which may exist as a result of being carried into the room by personnel, work articles, or equipment. Basically, the ionization devices consist of a group of electrostatically charged bars arranged in a generally planar configuration, with some form of ground grid disposed on opposite sides of the charged bars with respect to the direction of movement of the air. Normally, the entire air stream is subject to the influence of this assembly.